A display for displaying images is one of light-emitting elements necessary in modem life, which takes various configurations, such as so-called TV monitors, liquid crystal displays that have been developed rapidly in recent years, and EL (Electro Luminescence) displays that are expected to develop future, to meet requirements. Above all, an organic EL display has been most attracting attention as a next-generation flat panel display device.
In the light-emission mechanism of a light-emitting element composing an organic EL display, by locating a light-emitting layer composed of a composition that has a light-emitting property between electrodes and applying a current, an electron injected from a cathode and a hole injected from an anode are recombined in a luminescent center of the light-emitting layer to form a molecular exciton, and a photon emitted when the molecular exciton returns to the ground state is used. Therefore, injecting a hole and an electron into an organic thin film efficiently is one of requirements for manufacturing a high-efficiency light-emitting element.
In an operating condition of an electroluminescent element, typically, a current around 100 mA/cm2 is injected into a basically high electric resistance organic thin film. In order to realize the high-density current injection like this, it is necessary to reduce a hole-injection barrier from an anode and an electron-injection barrier from a cathode as mush as possible. Namely, a metal that has a small work function must be used as the cathode while an electrode that has a large work function must be selected as the anode. As for the cathode, by selecting various metals or alloys, the work function can be controlled virtually at will. On the contrary, in a general light-emitting element, what happening now is that an anode is limited to a transparent conductive oxide since the anode is required to have transparency, and in consideration of stability, transparency, resistivity, and the like, it is inevitable that several oxide conductive films typified by indium-tin oxide (hereinafter, referred to as ITO) are selected at this time. The work function of an ITO film can be changed to be made larger by a background during deposition or surface treatment. However, the method like this has limitations. This blocks reducing the hole-injecting barrier.
As one of methods for reducing a hole-injection barrier from an ITO anode, it is known to insert a buffer layer on the ITO film. By optimizing the ionization potential of the buffer layer, the hole-injection barrier can be reduced. The buffer layer like this is referred to as a hole injecting layer. As representatives of ones that function as a hole injecting layer, conjugated polymers can be cited. As typical examples, conjugated polymers such as polyanilines (Non-Patent Reference 1) and polythiophene derivatives (Non-Patent Reference 2) are known. By using the material as a hole injecting layer, the hole-injection barrier is reduced, and holes are efficiently injected, so that a light-emitting element is improved in efficiency and life, and the driving voltage can also be reduced.
These polymer materials, in which a sufficient hole injecting property cannot be achieved by only the conjugated polymers, always need a dopant for enhancing a hole injecting property. As the dopant, strongly acidic materials such as poly (styryl sulfonic acid) and camphorsulfonic acid are often used. However, there is a possibility that these strongly acidic materials have harmful effects on a transistor for driving a light-emitting element, for example, in an active matrix display device. Further, these polymer materials generally dissolve only in water or a solvent with high polarity. Therefore, after a polymer solution manufactured by dissolving the conjugated polymer material described above in these solvents is applied to a substrate, it is necessary to remove the solvent with a high boiling point at a high temperature under reduced pressure. In this method, it is not always easy to remove the solvent, in particular, water, thus it is conceivable that degradation of an element is accelerated by remaining solvent.
Based on these technological back ground, a hole injecting polymer that needs no dopant for enhancing a hole injecting property and is soluble in an organic solvent with a low boiling point, that is, a low polarity.
Non-Patent Reference 1: Y. Yang, et al., Appl. Phys. Lett. 1994, 64, 1245-1247
Non-Patent Reference 2: S. A. Carter, et al., Appl. Phys. Lett. 1997, 70, 2067-2069